The present invention relates to downhole drilling of subterranean formation. More particularly, this invention relates to the determination of downhole forces on a drilling tool during a drilling operation.
FIG. 1 shows a drilling rig 101 used to drill a borehole 102 into an earth formation 103. Extending downward from the rig 101 is a drill string 104 with a drill bit 105 positioned at the bottom of the drill string 104. The drill string also has a measurement-while-drilling (“MWD”) tool 106 and a drill collar 107 disposed above the drill bit 105.
The drill bit and associated sensors and equipment that are located near the bottom of the borehole while drilling form the Bottom Hole Assembly (“BHA”). FIG. 2 shows a BHA 200 positioned at the bottom of a borehole 102. The drill bit 105 is disposed at the end of the drill string 104. An MWD tool 106 is disposed proximate to the drill bit 105 on the drill string 104, with a drill collar 107 positioned proximate to the MWD tool 106. FIG. 2 shows sensors 202 disposed about the drilling tool for taking various downhole measurements.
The drilling of oil and gas wells involves the careful manipulation of the drilling tool to drill along the desired path. By determining and analyzing the forces acting on the drilling tool, decisions may be made to facilitate and/or improve the drilling process. These forces also allow a drill operator to optimize drilling conditions so a borehole can be drilled in a more economical way. The determination of the forces on the drill bit is important because it allows an operator to, for example, detect the onset of drilling problems and correct undesirable situations before a failure of any part of the system, such as the drill bit or drill string. Some of the problems that can be detected by measuring these downhole forces include, for example, motor stall, stuck pipe, and BHA tendency. In cases where a stuck pipe occurs, it may be necessary to lower a ‘fishing’ tool into the wellbore to remove the stuck pipe. Techniques involving tools, such as drilling jars, have been developed to loosen a BHA stuck in the borehole. An example of such a drilling jar is described in U.S. Pat. No. 5,033,557 assigned to the assignee of the present invention.
The forces acting on the drilling tool that can affect the drilling operation and its resulting position may include, for example, weight-on-bit (“WOB”) and torque-on-bit (“TOB”). The WOB describes the downward force that the drill bit imparts on the bottom of the borehole. The TOB describes the torque applied to the drill bit that causes it to rotate in the borehole. A significant issue during drilling is Bend, the bending of the drill string or bending forces applied to the drill string and/or drill collar(s). Bend can result from WOB, TOB, or other downhole forces.
Techniques have been developed for measuring the WOB and the TOB at the surface. One such technique uses strain gauges to measure forces on the drill string near the drill bit. A strain gauge is a small resistive device that is attached to a material whose deformation is to be measured. The strain gauge is attached in such a way that it deforms along with the material to which it is attached. The electrical resistance of the strain gauge changes as it is deformed. By applying an electrical current to the strain gauge and measuring the differential voltage across it, the resistance, and thus the deformation, of the strain gauge can be measured.
An example of a technique using strain gauges is described in U.S. Pat. No. 5,386,724 issued to Das et al (“the Das patent”), assigned to the assignee of the present invention. The Das patent discloses a load cell constructed from a stepped cylinder. Strain gauges are located on the load cell, and the load cell is located in a radial pocket in the drill string. As the drill string deforms due to downhole forces, the load cell is also deformed. The strain gauges on the load cell measure the deformation of the load cell, which is related to the deformation of the drill collar. As described in the DAS patent, the load cell may be inserted into the drill collar so that the load cell deforms with the drill collar.
FIGS. 3A and 3B show the load cell 300 disclosed in the Das patent. The load cell 300, as shown in FIG. 3A, has eight strain gauges located on the annular surface 301. The strain gauges include four weight strain gauges 311, 312, 313, and 314, and four torque strain gauges 321, 322, 323, and 324. The weight strain gauges 311-314 are disposed along the vertical and horizontal axis, and the torque strain gauges 321-324 are disposed in between the weight strain gauges 311-314. FIG. 3B shows the load cell 300 disposed in a drill collar 331. When the drill collar 331 is deformed as a result of downhole forces, the load cell 300 disposed in the drill collar is also deformed, allowing the deformation to be measured with the strain gauges.
Other examples of load cells and/or strain gauges may be found in U.S. Pat. No. 5,386,724 and pending U.S. patent Ser. No. 10/064,438, both assigned to the assignee of the present invention. Load cells typically can be constructed of a material that has very little residual stress and is more suitable for strain gauge measurement. Many such materials, may include for example INCONEL X-750, INCONEL 718 or others, known to those having skill in the art.
Despite the advances in strain gauges, there remains a need to provide techniques capable of taking accurate measurements under severe downhole drilling conditions. Conventional sensors are often sensitive to bending about the drill collar axis. Additionally, conventional sensors are often sensitive to temperature fluctuations often encountered in the wellbore, such as gradients over the wall of the drill collar at the sensor location and uniform temperature rises from ambient temperature.
It is desirable that a system be provided that is capable of eliminating interferences generated by forces acting on the drill string between the drill bit and the surface. It is further desirable that such a technique magnify the deformations received for ease of measurement and/or manipulation. It is preferable that such a system be capable of operating with sufficient accuracy despite temperatures fluctuations experienced in the drilling environment, and of eliminating the effects of hydrostatic pressure on measurement readings. The present invention is provided to address the need to develop systems capable of improving measurement reliability resulting from wellbore interference, mounting problems and/or temperature fluctuations, among others.
What is still needed, however, is a more accurate and reliable load sensor with a long working life that is not affected by downhole working conditions.